1. Field of the Invention
This invention relates to improvements in methods and apparatuses for controlling the phase and amplitude of an RF (acoustic) carrier which is modulated with a similar frequency pulsed signal and, more particularly, to improvements in acousto-optic control circuitry to provide uniform laser printing.
2. Description of the Related Art
Laser printing technology provides a pulsed imaging raster output scanner for providing an acoustic carrier which can be modulated with a video signal, prompting a laser light output to image an optical grating onto a photoconductor. The video signal and the RF carrier are processed to provide a single modulated carrier. Typically, the video signal is a digital pulse which, in the logically high state, triggers a burst of RF energy to an acousto-optic (AO) modulator for activating a crystal within the modulator whereupon, when the RF carrier is passed into the crystal by a transducer, the RF carrier becomes an acoustic carrier. When a region of the crystal within the AO modulator is activated, a laser light beam is passed through that region and imaged onto a photoconductive surface to photodischarge a pixel site. When the video signal goes to the logically low state, the RF carrier is not allowed to pass energy to the AO modulator. Thus, the crystal within the modulator is deactivated, and the laser light beam is turned-off until the next activation by a logically high video signal.
The video frequency of a raster output scanner is typically lower than the acoustic carrier frequency, but for high speed raster output scanners, the frequency of the video signal can approach the frequency of the acoustic carrier. For example, in one color printing system, the minimum pixel time available during scanning is 11.4 nanoseconds whereas the period of the RF signal is approximately 20 nanoseconds, so that approximately two periods of the video signal occur during each period of the acoustic waveform. When the frequency of the video signal and the frequency of the acoustic carrier are similar to one another, beat-like artifacts appear in the optical pattern that is output by the AO modulator, resulting in non-uniform printed pixels that vary in appearance periodically with the phase of the acoustic carrier, such that the width and density of the printed pixels are inconsistent. The present invention adjusts the phase of the carrier signal to eliminate the beat-like artifacts.
In U.S. Pat. No. 4,639,073 to Yip et al, an electro-optic pulse imaging raster output scanner is described that includes means for pulsing modulator electrodes at a speed equal to the speed of a scanning polygon and in a direction opposite to the polygon scanning direction to obviate image blur. Yip et al recognizes enhanced image clarity and reduced image blurring by matching the velocity of the acoustic pulse with the velocity of the spot on a recording member where a beam is focused. (In the literature of the field, this is referred to as a "pulsed imaging scanner."). However, phase synchronization between video and acoustic signals having similar, but not identical, frequencies at the start-up time of the modulator, is not described or suggested in the reference.
A synchronizing signal for providing clear printing by controlling the modulation starting time and ending time of a laser beam which scans on a photosensitive drum is described in Japanese Patent Application No. 58-13069 to Nagaoke. The Width of a square wave synchronizing signal establishes the duration of a laser beam. In Japanese Patent Application No. 60-114071, a device for generating a light beam scan synchronizing signal is described for starting a light beam scan upon the fall of a signal outputted from a comparator. In both Japan 58-13069 and Japan 60-114071, there is no description or suggestion of synchronizing the phase of a video and acoustic signal having similar but not identical frequencies in order to affect beat elimination in an acoustooptic modulator.